Such methods and arrangements are rather well known at present in connection with producing optical cables. Examples of known solutions include arrangements described in U.S. Pat. No. 4,893,998, German patent 3,425,649 and European patent application 0,286,819.
The disadvantages of the aforementioned arrangements include for example that the direction of travel of the unit formed by the fibres and the tubular sheathing changes rather rapidly after the extrusion head, since the unit is passed around a so-called locking wheel. In a production line equipped with a locking wheel the amount of the fibres is limited, since otherwise the fibres are positioned one upon another on the locking wheel, whereupon the outermost fibres may break at the worst. In order to operate reliably especially under conditions where the temperature fluctuates, for example an SZ stranded cable having a so-called minipipe structure requires great stability of each component of the cable and controlled interaction between the components. Examples of matters that cannot be realized in the best possible manner with the prior arrangements include the levelling of the variation in the length of the fibres situated inside the same protective pipe or sheathing, which is the most important factor in an SZ stranded multi-fibre structure, wherefore controlled levelling during the manufacture of the cable is important for the final result. The levelling of the variation in the fibre length is controlled by regulating the mutual tension of the fibres. Another example concerns the dimensions of the pipe or sheathing acting as a protective housing. A third example relates to the relaxation following the manufacture of an individual protective pipe or sheathing and the control of the relaxation during the manufacture. The stability must be absolute in an SZ stranded structure.